The present invention relates to a thin film actuated mirror array in an optical projection system and to a method for manufacturing the same, and more particularly to a thin film actuated mirror array in an optical projection system having a secondary supporting member between a substrate and a supporting layer so as to make an initial deflection of the actuator uniform, thereby increasing a contrast of a picture projected onto a screen, and to a method for manufacturing the same.
In general, light modulators are divided into two groups according to their optics. One type is a direct light modulator such as a cathode ray tube (CRT). The other type is a transmissive light modulator such as a liquid crystal display (LCD). The CRT produce superior quality pictures on a screen, but the weight, the volume and the manufacturing cost of the CRT increase according to the magnification of the screen. The LCD has a simple optical structure, so the weight and the volume of the LCD are less than those of the CRT. However, the LCD has a poor light efficiency of under 1 to 2% due to light polarization. Also, there are some problems in the liquid crystal materials of the LCD such as sluggish response and overheating.
Thus, a digital mirror device (DMD) and actuated mirror arrays (AMA) have been developed in order to solve these problems. Currently, the DMD has a light efficiency of about 5%, but the AMA has a light efficiency of above 10%. The AMA enhances the contrast of a picture on a screen, so the picture on the screen is more apparent and brighter. The AMA is not affected by and does not affect the polarization of light and therefore, the AMA is more efficient than the LCD or the DMD.
FIG. 1 shows a schematic diagram of an engine system of a conventional AMA which is disclosed in U.S. Pat. No. 5,126,836 (issued to Gregory Um). Referring to FIG. 1, a ray of incident light from a light source 1 passes a first slit 3 and a first lens 5 and is divided into red, green, and blue lights according to the Red-Green-Blue (R-G-B) system of color representation. After the divided red, green, and blue lights are respectively reflected by a first mirror 7, a second mirror 9, and a third mirror 11, the reflected light is respectively incident on AMA devices 13, 15, and 17 corresponding to the mirrors 7, 9, and 11. The AMA devices 13, 15, and 17 tilt mirrors installed therein, so the incident light is reflected by the mirrors 7, 9, and 11. In this case, the mirrors 7, 9, and 11 installed in the AMA devices 13, 15, and 17 are tilted according to the deformation of active layers formed under mirrors. The lights reflected by the AMA devices 13, 15, and 17 pass a second lens 19 and a second slit 21 and form a picture on a screen (not shown) by using projection lens 23.
The AMA is generally divided into a bulk type AMA and a thin film type AMA. The bulk type AMA is disclosed in U.S. Pat. No. 5,469,302 (issued to Dae-Young Lim). In the bulk type AMA, after a ceramic wafer which is composed of multilayer ceramics inserted into metal electrodes therein is mounted on an active matrix having transistors, a mirror is mounted on the ceramic wafer by means of sawing the ceramic wafer. However, the bulk type AMA has disadvantages in that it demands a very accurate process and design, and the response of an active layer is slow. Therefore, the thin film AMA manufactured by using semiconductor technology has been developed.
The thin film AMA is disclosed in U.S. Ser. No. 08/336,021, entitled "Thin Film Actuated Mirror Array Used in an Optical Projection System and Method for the Manufacture Thereof," which is now pending in USPTO and is subject to an obligation to the assignee of this application.
FIG. 2 shows a cross-sectional view of the thin film AMA. Referring to FIG. 2, the thin film AMA has an active matrix 31, an actuator 33 formed on the active matrix 31, and a mirror 35 installed on the actuator 33. The active matrix 31 has a substrate 37, M.times.N (in which M, N is integers) number of transistors (not shown) which are installed in the substrate 37, and M.times.N (in which M, N is integers) number of connecting terminals 39 respectively formed on the transistors.
The actuator 33 has a supporting member 41 formed on the active matrix 31 which includes connecting terminal 39, a first electrode 43 having a bottom of first portion thereof attached to the supporting member 41 and having a second portion formed parallel to the active matrix 31, a conduit 49 formed in the supporting member 41 so as to connect connecting terminal 39 to the first electrode 43, an active layer 45 formed on the first electrode 43, a second electrode 47 formed on the active layer 45, a spacing member 51 formed at first portion of the second electrode 47, and a supporting layer 53 having a bottom of first portion thereof attached to the spacing member 51 and having a second portion formed parallel to the second electrode 47. The mirror 35 is installed on the supporting layer 53.
A manufacturing method of the thin film AMA will be described below. FIGS. 3A to 3D illustrate manufacturing steps of the thin film AMA. In FIGS. 3A to 3D, the same reference numerals are used for the same elements in FIG. 2.
Referring to FIG. 3A, at first, the active matrix 31 which includes the substrate 37 in which M.times.N number of transistors (not shown) are formed and M.times.N number of connecting terminals 39 respectively formed on the transistors is provided. Subsequently, after a first sacrificial layer 55 is formed on the active matrix 31, the first sacrificial layer 55 is patterned to expose a portion of the active matrix 31 where the connecting terminal 39 is formed. The first sacrificial layer 55 can be removed by using an etching method or by means of chemicals.
Referring to FIG. 3B, the supporting member 41 is formed on the exposed portion of the active matrix 31 by a sputtering method or a chemical vapor deposition (CVD) method. Next, after a hole is formed through supporting member 41, the conduit 49 is formed in the supporting member 41 by filling the hole with an electrically conductive material, for example tungsten (W). The conduit 49 electrically connects the connecting terminal 49 to the first electrode 43 which is successively formed. The first electrode 43 is formed on the supporting member 41 and on the first sacrificial layer 55 by using an electrically conductive material such as gold (Au) or silver (Ag). The active layer 45 is formed on the first electrode 43 by using a piezoelectric material, for example lead zirconate titanate (PZT). The second electrode 47 is formed on the active layer 45 by using an electrically conductive material such as gold (Au) or silver (Ag).
The transistor installed in the active matrix 31 converts a picture signal which is caused by an incident light from a light source into a signal current. The signal current is applied to the first electrode 43 through the connecting terminal 39 and the conduit 49. At the same time, a bias current from a common line (not shown) formed on the bottom of the active matrix 31 is applied to the second electrode 47, so an electric field is generated between the second electrode 47 and the first electrode 43. The active layer 45 formed between the second electrode 47 and the first electrode 43 is actuated according to the electric field.
Referring to FIG. 3C, after a second sacrificial layer 57 is formed on the second electrode 47, the second sacrificial layer 57 is patterned to expose a portion of the second electrode 47 adjacent to a portion under which the supporting member 41 is formed. After the spacing member 51 is formed at the exposed portion, the supporting layer 53 is formed on the second sacrificial layer 57 and on the spacing member 51. Also, the mirror 35 for reflecting the incident light is formed on the supporting layer 53.
Referring to FIG. 3D, the mirror 35, the supporting layer 53, the second electrode 47, the active layer 45 and the first electrode 43 are sequentially patterned so that M.times.N number of pixels having predetermined shapes are formed. Consequently, after the first sacrificial layer 55 and the second sacrificial layer 57 are removed, pixels are rinsed and dried in order to complete the thin film AMA.
However, in the above-described thin film AMA, due to the thickness variations within a processed wafer and due to the uneven residual stresses and stress gradients of the thin film composites within a single pixel, a substantial amount of initial deflection of the actuator right after the release of the sacrificial layer exists. Since the initial deflection of an actuator directly affects the brightness of the corresponding image pixel, the uniformity of the initial tilting among the actuators is required. The deflection of the thin film AMA can be induced by two kinds of bending moments. One is present at the step-up boundary formed along the width-wise (W) of the actuator, and the other is distributed along the length-wise (L) of the actuator. The boundary bending moment is primarily dependent on the average residual stress and stress gradient, while the length-wise bending moment is mainly dependent on the stress gradient but have no dependence on the average residual stress. That is, the step-up region deflection by the boundary bending moment is dominant over the length-wise deflection by the length-wise bending moment.
The step-up boundary bending moment is responsible for 70-80% of the total deflection.
Due to the step-up boundary bending moment, the contrast of the picture projected onto a screen is decreased.
Knowing that the residual stress at the step-up region is the dominant factor for the initial deflection, there are two ways of controlling the initial deflection. One is to control the residual stress which is the result of the PZT shrinkage at the step-up region and the other is to reinforce the structure less sensitive to the pixel-to-pixel residual stress deviations and to the stress gradients. The process parameter variations within a wafer, which are common in semiconductor fabrication processes, make it difficult to achieve the acceptable uniformity of the dimensions and the residual stress variation below .+-.5%. Therefore, even the best control of the process results in inevitable pixel-to-pixel non-uniformity within a wafer. The improvement in structural design should be accomplished together with the best effort of process parameter control to achieve uniform and minimum initial deflections.